Human monocytes treated with cycloheximide (CHX) demonstrated a dose- and- time dependent inhibition of PGE2 synthesis and release in response to stimulation with PMA, ionomycin, serum-treated zymosan, or Con A. The effect of CHX required preincubation and was largely reversible within two hours. Thromboxane A2 release was similarly affected but no comparable effects were observed on labeled arachidonic acid release or LTB4 generation. The PGE2 response in the presence of CHX was also inhibited when monocytes were given exogenous arachidonic acid with or without stimulation. CHX pretreatment also comparably decreased the amount of immunoreactive cyclooxygenase in resting and stimulated monocytes. These data indicate that monocyte cyclooxygenase, in contrast to phospholipase A2 or 5-lipoxygenase and their regulatory proteins, turns over rapidly and may be a target for up-or-down- regulation by pharmacologic, or (potentially) physiologic agents which affect protein synthesis.